In synthetic aperture radar the motion of the aircraft is employed to achieve greater angular resolution than obtainable by antennas that can be mounted on the aircraft. The motion of the aircraft permits formation of a synthetic antenna that is larger in size than the real antenna. This large synthetic antenna has an angular resolution corresponding to the angular resolution of an equally large physical antenna. The aircraft flies in a predetermined path and repeatedly transmits a radar signal. It is typical for this radar signal to be directed to the side of the flight path via a small antenna. This cross track view may be directly perpendicular to the flight path or at some angle less than perpendicular. The same antenna is employed to receive return echoes of the transmitted signal.
The return echo signals are processed in two dimensions. The time to receive an echo return from any particular piece of terrain corresponds to its slant range from the aircraft. This range is called slant range because it typically follows a downwardly slanting path from the aircraft to the ground. The echo returns also have differing Doppler frequencies. The motion of the aircraft imparts a Doppler frequency shift in the echo returns from the stationary ground. Areas directly ahead of the aircraft have a maximum closing Doppler frequency, those directly behind have a maximum opening Doppler frequency. Areas at varying angles between these extremes have intermediate Doppler frequencies. The combination of time of return and Doppler frequency permit production of a two dimensional feature map of individual resolution cells in slant range and Doppler frequency. Plural echo returns can be processed together with the known path and velocity of the aircraft to produce a composite picture of the area imaged.
There is a problem with this technique. This technique cannot distinguish non-surface from surface objects within the same slant range and Doppler frequency resolution cell. An echo signal within a resolution cell having a particular time of return and Doppler frequency does not define a unique location. Regions of echo return times equal within the limits of measurement lie on a spherical shell centered at the antenna location. Regions of Doppler frequencies equal within the limits of measurement lie on a conical shell having its vertex at the antenna and its axis on the velocity vector of the aircraft. The intersection of these regions forms a vertically disposed circular annulus. Actual echo returns can only come from areas illuminated by the transmissions, so that return areas are further limited to the solid angle cone of the antenna. Still this leaves an ambiguity in the location forming the echo return. Thus a non-surface object, such as an underground object or a flying object, cannot be distinguished from the surface return in the same resolution cell. This is true even though the non-surface object occupies a different physical space than the interfering surface return.